Structure sensitivity in gas sorption and conversion on metal-organic frameworks

Many catalytic processes depend on the sorption and conversion of gaseous molecules on the surface of (porous) functional materials. These events often preferentially occur on specific, undercoordinated, external surface sites. Here we show the combination of in situ Photo-induced Force Microscopy (PiFM) with Density Functional Theory (DFT) calculations to study the site-specific sorption and conversion of formaldehyde on the external surfaces of well-defined faceted ZIF-8 microcrystals with nanoscale resolution. We observed preferential adsorption of formaldehyde on high index planes. Moreover, in situ PiFM allowed us to visualize unsaturated nanodomains within extended external crystal planes, showing enhanced sorption behavior on the nanoscale. Additionally, on defective ZIF-8 crystals, structure sensitive conversion of formaldehyde through a methoxy- and a formate mechanism mediated by Lewis acidity was found. Strikingly, sorption and conversion were influenced more by the external surface termination than by the concentration of defects. DFT calculations showed that this is due to the presence of specific atomic arrangements on high-index crystal surfaces. With this research, we showcase the high potential of in situ PiFM for structure sensitivity studies on porous functional materials.


Supplementary Figure 1.
Experimental approach for crystal-plane specific analysis. a) Highly defined ZIF-8 crystals provide facile identification of crystal planes in b) hyperspectral images at varying formaldehyde pressure. c) Measurement-specific crystal plane maps were drawn for the construction of masks. d, e, and f) gives examples of a mask used for a facet, edge, and corner, respectively. Spectra of individual masks were analyzed for outlier behavior before averaging plane-dependent masked spectra to improve the S/N ratio.

Density Functional Theory calculations
Surface models cleaved, giving rise to various under-coordinated sites. In our model, we assumed that surface terminations were always neutral as was done in the work of Weng and Schmidt, i.e., charged surface terminations are not considered as they require diffusive double layer of ions from solution to maintain overall charge neutrality of the system. [3] It has been shown that surface Zn-sites can be unsaturated or capped by various terminating groups, such as hydroxyl, carbonate, and monodentate 2-methylimidazole (Mim), depending on the system conditions. [3,4] The theoretical work of Weng and Schmidt suggests that the most plausible surface terminations are primarily imidazole-related species. [3] Thus, in all our models we saturated dangling Zn-N bonds with protonated 2-methylimidazole (Hmim) with the aim to elucidate general structural characteristics of various ZIF-8 facets, independently of system conditions. For comparison, we also considered zero coverage of surfaces.
In the next step, adsorption of formaldehyde was studied by adsorbing one furfuryl alcohol molecule on each side of the clean surface.
To create a defective surface models, we replaced one terminal Mim with a pyrrole. A cleavage of the surfaces lead to a large number of surface sites, and it is impossible to systematically examine the stability of the pyrrole linker with respect to all possible positions of a defect site. Instead, by analyzing the local structure of each surface site we identified a set of surface motifs, shown in Supplementary Figure 3, which serve as universal descriptors of the surface terminations. In the next step, we then introduced a defect site by replacing a Mim linker with pyrrole on each of the surface motifs. We did so for both cleavages of {100} and {110} facets and for the surface sites on the most stable cleavage of the {310} plane. The surface was subsequently re-optimized. The adsorption of formaldehyde followed by the adsorption of water and second formaldehyde was also modelled in the same manner as for pristine surfaces.
Computational details DFT calculations were conducted using version 6.1. of CP2K software, within Gaussian and plane wave (GPW) approach. [5] GTH pseudopotentials combined with TZVP-MOLOPT basis set and plane-wave density cutoff 700 Ry were employed. The target convergence of SCF cycle was set to 10 -6 . All geometries were optimized using PBE exchange-correlation functional with a semi-empirical Grimme D3 correction. [6,7] Initially, the bulk structure together with the unit cell was optimized and after that unit cell dimensions were kept fixed.
The optimized unit cell dimensions were 17.069 Å in each direction, i.e. a cubic representation of the bulk ZIF-8 was used. Subsequently, all possible {100}, {110} and {310} surface terminations were constructed, and the structure was re-optimized.

Supplementary Table 1.
The unit cell parameters, which were used for the models of various surface orientations and cleavages. The naming system corresponds to the one presented in Supplementary Figure 3. The unit cell parameters of each slab are summarized in Table S1. Each of the surfaces was approximated by symmetric two-dimensional periodic slab models separated from each other in a perpendicular direction by a layer of vacuum at least 15 Å thick. To avoid interaction of the two surfaces within the same slab, the convergence of the slab thickness with respect to surface energy was tested. Based on the results, the slab depth of at least five Zn layers was chosen for each of the models. During the optimization, the middle Zn layers were fixed, while at least the top two Zn layers were allowed to relax. The thermodynamically most stable surface was the one with minimal surface energy, which was computed for a saturated surface as: and for an unsaturated (clean) surface as: Eslab is the energy of the symmetric slab with a total surface area of 2A and N and In defective models, the concentration of the pyrrole was always fixed to one defect per surface (i.e. two pyrroles per unit cell). Due to varying unit cell size with respect to the surface model it was not possible to compare surface energies in order to compare their stability. Instead, reaction energy was used: Where − is the energy of a defective slab and and are the gas phase energies of neutral pyrrole and neutral methyl-imidazole in the gas-phase. The factor ½ originates from the symmetry of the slab.
The adsorption of formaldehyde (FA) was similarly expressed in terms of reaction energy required to replace one Hmim per surface with the reactant molecule: where − is the energy of the original surface and is the energy of the adsorbent in the gas-phase. The goal of an introduction of the reaction energy is to allow a straightforward comparison of the stability of various adsorbates on both a pristine and a defective surface.
A schematic representation of various surface motifs. The colored scheme shows for which surface orientation and a cleavage height is the given surface motif present, e.g. motif V can be found only on 310-2 surface (Supplementary Figure 2). In the saturated surface model are empty terminal sites, visualized by a blue ellipse, filled with a capping Hmim.  Table S2. Consistent with AFM observations (Supplementary Figure 10), we find that the {110} surface is the most stable (Υ sat = -4.91 meV/Å 2 ), followed by {100} surface (Υ sat = -3.21 meV/Å 2 ) and {310}

Supplementary
which is the least stable (Υ sat = -2.67 meV/Å 2 ). The higher energy cuts display surface energies between -1.05 meV/Å 2 (310-2, Figure 3) and 3.45 meV/Å 2 (100-1, Figure 3). Each of the studied surfaces is terminated with different surface motifs visualized in the Supplementary Figure 3. We find no apparent correlation between the number of terminal linkers or density of surface sites and surface energy.  kJ/mol and 97.1 kJ/mol. When FA coordinates to the defective surface, the same OFA-ZnZIF and CFA-NZIF adducts as in the pristine case are formed, but the linker will be no longer coordinated to the Zn atom, since pyrrole's only N atom is bound to FA ( Figure 5). This will lead to the formation of undercoordinated Zn sites displaying Lewis acidity which we propose to be responsible for the conversion of formaldehyde on defective ZIF-8 surfaces. [11,12] Reaction

Structure sensitivity in heterogeneous catalysis
The performance of a functional material is often determined by only a small percentage of its surface sites and their atomic configuration. This phenomenon, known as structure sensitivity, describes the relationship between exposed crystal surfaces and rate of conversion and is well known in the field of heterogeneous catalysis. [13][14][15] This phenomenon occurs when distinct surface sites possess a different reactivity. [16] Changes in dispersion between surface sites thus results in a change in overall activity. [17] Supplementary Figure 7.
Schematic describing structure sensitivity in supported nanoparticles (NPs), and in porous functional materials. a) Three general types of structure sensitivity are recognized for supported NPs, where a relationship between intrinsic conversion rate and NP size is found based on which type of bond (sigma/pi) needs to be activated during the rate determining step.
Adapted from [13] b) Different types of surface sites/ensembles are required for σ/π which can be found on different crystal planes. c) The dispersion of these crystal planes depends on the size of the metal NPs. d) Structure sensitivity for porous functional materials, e.g. ZIF-8, describes the crystal plane-dependent sorption and conversion energies of adsorbates.
Adapted from [13,18]. e) For each crystal plane, multiple cuts can be made resulting in crystal Different types of structure sensitivity apply to specific chemical reactions. For example, in type ІІ structure sensitivity, conversion is limited by the rate of π-bond activation which specifically occurs over highly active step-edge or kink sites. [20] These sites require ensembles of atoms and their fraction increases with NP size, up to a material-dependent maximum. Similarly, type І structure sensitivity describes the decrease in intrinsic rate with NP diameter due to a loss of isolated unsaturated sites for σ-bond activation. [16] Furthermore, for other chemical reactions this size-dependency is absent leading to the concept of structure insensitivity (type ІІІ). [21] While for dense, non-porous metal nanoparticles it is relatively easy to distinguish between bulk atoms and the wide variety of distinct surface atoms, this distinction is more difficult for porous functional materials, such as zeolites and metal-organic frameworks (MOFs) due to their inner porosity (Supplementary Figure 7d-f). [22,23] However, analogous to metal nanoparticles, it is often observed for porous functional materials that their functionality is concentrated on their outer surface, and not in their inner porosity. [4] As a result, some evidence of similar structure sensitivity behavior for porous functional materials can be found in literature. [24][25][26][27] Traditionally, structure sensitivity studies have been limited to surface science techniques, such as Scanning Tunneling Microscopy (STM) or Low-Energy Electron Diffraction (LEED), as it can be applied to (low index) single crystal samples of metals. [28,29]

Masking of hyperspectral images
Using PiFM, we were not limited by the diffraction spot of IR, since the tip acts as an antenna for IR light, amplifying the signal from a nm-size spot on the sample. We were thus able to obtain an IR spectrum for every pixel of the microscopy images reported in the paper, together with the corresponding topographic image of the same crystal. This was crucial, since it allowed us to describe both the surface-averaged spectrum of the ZIF-8 crystal surface, by averaging the IR spectra of all these pixels, as well as allow us to separate spectral contributions of different crystal planes. To do so, we used the topography map, which was acquired simultaneously with the IR image, to create masks corresponding to a crystal plane, and finally averaged spectra in such regions.
We divided the acquired hyperspectral images into relevant areas (facets, edges, corners) to

In situ PiFM on pristine ZIF-8
Contour plots in situ formaldehyde sorption on pristine ZIF-8 Supplementary Figure 16. Mask-averaged spectra corresponding to the masks shown in Supplementary Figures 12-14.
The hyperspectral image was recorded for a pristine ZIF-8 crystal at 180 ppm of formaldehyde pressure.
Pressure-dependent crystal plane averaged IR spectrum

Supplementary Figure 18.
Crystal plane-averaged IR spectra as a function of increasing formaldehyde pressure for a pristine ZIF-8 crystal. Crystal plane-averaged spectra were constructed by averaging all maskaveraged spectra of each crystal plane. This was done to improve the S/N ratio and to facilitate inter-plane spectrum comparison. This information was used to construct Figure 2F-I.
In situ plane averaged difference spectra

Supplementary Figure 19.
Pressure-dependent difference spectra for the crystal planes of a pristine ZIF-8 crystal, where the spectra of crystals in nitrogen were subtracted from the spectra of crystal planes in formaldehyde. The difference spectra highlight the rise of formaldehyde bands at increasing VOC pressure and the faster response of high-index planes, such as the {310} corner planes.
This information was used to construct Figure 2I.

Mapping of intra-facet heterogeneity
Additional point spectra within PiFM maps

Supplementary Figure 20.
Additional point spectra taken at 300 ppm FA pressure within the IR maps shown in a, b) and Full spectra normalized to the highest band intensity corresponding to the PCA spectra shown in Figure 4G. Bands marked by the grey boxes belong to vibrations of the defective pyrrole linker. Peak ratios between various ZIF and pyrrole bands were calculated for each of the spectra.

Size analysis intra-facet nano-islands
Size analysis was performed in Fiji using the NanoDefine plugin "Particle Sizer" (PS) analyzer. [43] The PS script was developed to automatically measure the distributions of the characteristic size and shape properties of a nanomaterial. Compared to usual particle analysis in ImageJ, where a threshold is applied to the global image, the plugin uses a local threshold, estimated for a specific circular region with the configured radius. This allows for a better discrimination of even partially overlapping domains.
The domain distribution for the defect-engineered ZIF-8 crystal was evaluated from the clustered image reported in Figure 4F, which was converted to grey scale and analyzed using However, the pressure at which these IR bands appeared differed between crystal planes. Segmentation of some {100} planes over increasing formaldehyde pressure. The planes were segmented into defect-rich (red) and defect-poor (blue) areas. The spectra corresponding to these segments are plotted below. Using these spectra, the intra-plane influence of defect sites on formaldehyde sorption and conversion could be found.

Principal Component Analysis and clustering of hyperspectral images of defective ZIF-8
Supplementary Figure 30.
Examples of clustered hyperspectral images (created through Principal Component Analysis and clustering) of defective ZIF-8 crystals at increasing formaldehyde pressure. The spectra belonging to the clusters are shown below. PCA and clustering was applied to hyperspectral image to identify defect-rich and defect-poor regions. This approach was used to find defect regions spanning the whole crystals, rather than being confined to specific crystal planes. To find these regions we calculated ZIF/pyrrole peak ratios for all clusters. The highest and lowest ratio per pressure is plotted together with the bulk value found for the defective crystal in Figure 4E, I. Higher overall values were found since a crystal with high coverage with high-index planes was selected to increase spectrum quality for these planes. This information was used to construct Figure 4C.

DFT calculations on formaldehyde sorption on defective ZIF-8 surfaces
Supplementary Figure 31.

FA adsorption on ZIF-8 followed by in situ Attenuated total reflectance (ATR) FTIR spectroscopy
To study FA adsorption on ZIF-8 crystals at the bulk scale, ZIF-8 powders and thin films were deposited on ATR crystals and exposed to FA in situ. ATR configuration was chosen because transmission FTIR spectroscopy of ZIF-8 pellets showed strong interferences with the water and FA vapor, yielding low quality data.
Preparation of ZIF-8 samples on Si ATR crystals ZIF-8 samples were prepared by LbL synthesis according to the procedure described in the first section of the SI (see MOF deposition), with the only difference that Si ATR crystals were used instead of Au substrates.
To allow for more sample to be deposited on the ATR crystals and enhance the signals from Additionally, a commercial ZIF-8 sample (ACS materials, 500 nm crystal size) was used as a reference.
To prepare the samples for ATR experiments, the synthesized or commercial ZIF-8 powder was suspended in methanol, drop-casted on a Si ATR crystal and dried overnight at 50 °C. For calibration of FA and methanol vapours, transmission spectra were integrated in the C-H stretching region between 3100-2800 cm -1 . Concentrations were obtained from the band areas using reference spectra of 1 ppm/m FA or methanol from the PNNL database.
Before each FA adsorption experiment, the cell was flushed with pure N2 for 10 min.
Subsequently, the FA partial pressure was increased by 0.02 steps in a range of p/p0 = 0 -0.1, and in intervals of 0.1 for higher pressures, and kept for 3 min at each step to reach equilibrium.
Methanol adsorption isotherm measured via in situ FTIR-ATR reveals gate opening of ZIF-8 ZIF-8 is known to undergo structural changes upon adsorption of vapours of alcohols, water and acetone, so-called "gate opening". [46,47] Such deformations result in an increase in the ZIF-8 pore volume after exposure to certain critical concentrations of guest molecule vapour, and were associated with S-shaped isotherms during adsorption, not only for ZIF-8, but also ZIF-4, ZIF-7, and ZIF-9. [47] For ZIF-8, the critical vapour concentration at which an abrupt increase in absorption and penetration of the porous volume was observed due to gateopening was reported as 10, 5, and 3 vol.% for methanol, ethanol and propanol respectively. [46,47] The critical pressure was further observed to decrease with increasing molecular weight of the alcohol, with butanol having a critical pressure of around 0.1 %. [47] Notably, all these concentrations are higher than the maximum FA pressure used in this study (0.05 vol.%). These observations strongly suggest that the signals observed during FA adsorption on ZIF-8, both with PiFM and FTIR-ATR, only stem from species adsorbed on the surface of the crystals, as the pressure is too low to induce gate opening. We therefore here assume that no gate opening takes place during our study.
Prior to FA adsorption, we wanted to gather evidence that the signals are exclusively obtained from the surface of the ZIF-8 crystals and no pore condensation and penetration of the FA takes place. Due to the low achievable FA concentration and the high partial vapor pressures needed to observe the characteristic jump in sorption isotherms upon pore condensation, methanol, as most similar molecule in terms of size and polarity, was used to achieve a broad range of vapor concentrations and to determine the pore condensation step. Supplementary  All considered, the ATR-IR results prove that bands associated with all species observed using PiFM can be found in the bulk experiments. We observed that the same species found at lower FA concentrations in PiFM experiments are present in the ATR experiments, followed by an increase in concentration of formates. However, while PiFM allows to pick a ZIF-8 crystal or even a certain crystal plain for FA adsorption and thereby ensure a controlled defect-free experiment, the exclusion of defective ZIF-8 crystals cannot be avoided in bulk measurements. Therefore, PiFM gives access to adsorption studies on defined crystal planes beyond standard FTIR spectroscopy.